PVC stack into cast iron cleanout tee into clay pipe

Discovered that I had a leak in the basement. Traced it to the main stack where it meets a cast iron clean out tee. There was a circular patch of cement that cracked and allowed water to flow out onto the basement floor. I used hydraulic cement to patch the area, not thinking about the joint between the cast iron and what I assume is clay tile. Within 2 days, the patch cracked and water leaked out on the floor. I chipped away the patch I made, and in order to see the problem better, I chipped away at the circular cement patch that has been there since I moved in 12 years ago.

What I ended up with is a large mouth (approximatley 6.5 inch) opening in the floor and the 4 inch cast iron tee. It appears that there was a clay collar or possible a clay hub that connected the 4 inch cast iron tee to the 6.5 inch mouth. I plan on replacing the cast iron tee with a new cast iron tee because I cannot open the clean out. But how I seal the opening from the 4 inch cast iron tee to the 6.5 inch mouth in the floor?
I will be using a 4 inch donut to seal the PVC to the cast iron. Any advice is greatly appreciated!